Hi,
sorry but this is not possible natively, you need to create a plugin that change the username from DB.
- Download, Install and Enable Skeleton Plugin (you will find it at http://docs.easy-profile.com/index.php/article/triggers)
- Open file /plugins/jsn/skeleton/skeleton.php
- remove samples functions: triggerFieldAvatarUpdate, triggerProfileUpdate
- add function like this:

public function triggerProfileUpdate($user,&$data,$changed,$isNew){ 

    

    $user_id = $user->id;

    $new_username = $data['field1']; // Replace field1 with the alias of your custom field

    

    $query = 'UPDATE #__users SET username = ' . $db->quote($new_username) . ' WHERE id = ' . $user_id;

    

    $db = JFactory::getDbo();

    $db->setQuery($query);

    $db->execute(); 

}

NOTE: This code is not tested and not supported, this is only for example purpose. You need to have a minimum of PHP knowledges